Your city seethes with ghosts. Its impossibly twisted streets stream with magic, and its chimneys exude smoke of a decidedly hallucinatory nature. Why do modern, urban places feel as if they are home to so many unexplainable, otherworldly forces?

History and the Unknowable

For every piece of city magic, there is some kind of real-world analogue. Take, for example, the urban vampire. She eats aspiring actresses in the show Angel; he stalks prey through every city in Europe and America in Anne Rice's vampire book series. They run their own goth clubs in the Underworld movies, and manage to find really nice Bristol flats in UK serial Being Human. Even Dracula wanted to move to London from the lonely Carpathians.

Why are vampires' mythical, bloodstained faces hidden in trashed alleyway shadows in your city? Because most cities aren't just packed with people. They are layered with history - sometimes many thousands of years of it. Even in relatively new cities like San Francisco or Toronto, several dead generations have walked the streets before you. They've lived in your houses, and gone to your favorite shops.

Like vampires, these unknowable phantoms of history are hidden. But their influence lingers in the present. When you go into the buildings they built, buy their used clothing at the Goodwill, and eat in the dining rooms where they once did, you brush shoulders with the dead.

Emergent Formations and Conspiracies

Cities are also packed with strange secret societies and conspiracies. Fritz Lieber's novel of city magic, Our Lady of Darkness, is a classic in this regard. He invents the term megapolisomancy - the magic of big cities - to describe a strange hex that has been cast on the entire city of San Francisco. A mystical triangle emerges when the Transamerica Pyramid is erected, forming the dark symbol with the toothy peak at Corona Heights and the apartment of a sorcerer. Cities that turn into mystical symbols occur in Alan Moore's graphic novel From Hell, where London has been laid out in a secret pentagram. In both stories, secret societies are responsible for both creating and reading these symbols, in order to gain power or destroy other people.

In "The Horror At Red Hook," H.P. Lovecraft imagines that the city of New York sits atop a vast underground series of rivers and caverns where foreigners consort with devils and threaten the whole stucture of civilization.

These conspiracies hold a fantastical mirror up to a true characteristic of urban life: Emergent formations. Out of many chaotic parts, neighborhoods and subcultures form in cities - seemingly without any design or plan. More frightening than a conspiracy is a coherence that comes out of nothing except random parts connecting. Immigrant neighborhoods come into being without design; corporations ooze into other corporations; riots erupt; artist subcultures take over buildings intended for general use. Indeed, in Our Lady of Darkness, the Transamerica Pyramid has replaced one such building subculture - a true urban historical fact that bleeds easily into city magic.

Surveillance and Doubling

Cities are covered in a million eyes - and they all watch each other. Windows look into windows; crowds are a morass of furtive glances; and surveillance cameras adorn every surface. We pretend we have privacy in cities, but always doubt it. There is something uncanny about this feeling. You split into two selves: The self you perceive yourself to be, and the self you perceive other people are always watching. Who are you? Are you what the elevator camera sees, or the person you see in the (possibly two-way) mirror?

If there are two versions of you, it makes sense that there is a secret other city beneath the surface of the one you inhabit. Or perhaps existing in the same space as your city, as the characters discover in China Mieville's haunting noir fantasy novel The City & The City. Or perhaps your city is riddled with passageways to another one, which you can enter only through mirrors. This is one of the many intriguing details in Susanna Clarke's novel Jonathan Strange & Mr. Norrell, where the newly-revived field of sorcery has revealed that magicians can get around England using a series of roads reached via mirrors. The roads themselves seem to lead through a vast and abandoned city.

And in Neil Gaiman's Neverwhere, another city lurks literally beneath London's feet – a city of magic and monsters, whose citizens share with London only a series of landmarks: the Tube stops. Even the Harry Potter series plays with this idea, creating an invisible train platform in King's Cross station that can only be reached by people with magic.

Population Density and Hallucination

Cities can be so vast that walking through them is akin to hallucinating. The road we just went down seems to have disappeared. There are three streets with almost exactly the same name, running parallel to one another. People in strange costumes are congregating on a street corner for no reason. Elsewhere, a sea of people in matching business suits fill the streets with a freakishly uniform charcoal grey.

The constantly-morphing city in Dark City, where our hero without memories awakens to discover himself the victim of a vast conspiracy, is a literal version of the everyday surrealism of city life. And so is the urban landscape in City of Lost Children, a steamy, mechanical world where a monster who cannot dream tries to extract the dreams of children. In Wicked City, the world of magics that spreads across comic books and an amazing live-action movie, is the ultimate hallucination. People merge into buildings – at one point, a demon turns into an elevator, and then a motorcycle.

The very fabric of our bodies becomes permeable in the hallucinatory city. Something about the incredible population density in cities makes this possible. We are surrounded by human-made objects as we wander the forests of buildings. Human dreams spray across the walls in the forms of murals, art, advertisements. We exist in the dreamscape of our own species. It is not a natural place. It is the place we built out of our own fantasies.